1. Field of the Invention
The present invention relates to a method of forming a semiconductor film having a crystal structure, on a substrate having an insulating surface, and also to a method of fabricating a semiconductor device which employs the semiconductor film as an active layer. More particularly, it relates to a method of fabricating a thin film transistor in which an active layer is formed of a crystalline semiconductor film. Incidentally, here in this specification, the expression xe2x80x9csemiconductor devicexe2x80x9d is intended to signify general devices which can function by utilizing semiconductor properties, and it shall cover within its category, an electro-optical device which is represented by a liquid crystal display device of active matrix type formed using thin film transistors, and an electronic equipment in which such an electro-optical device is installed as a component.
2. Description of the Related Art
There have been developed thin film transistors (hereinbelow, often abbreviated to xe2x80x9cTFTsxe2x80x9d) each of which employs as its active layer a crystalline semiconductor film prepared in such a way that an amorphous semiconductor film is formed on an insulating substrate having a light transmissivity, such as of glass, and then crystallized by laser annealing, thermal annealing or the like. A glass substrate of barium borosilicate glass, alumino-borosilicate glass or the like is often employed as the insulating substrate. Although such a glass substrate is inferior to a quartz substrate in the point of a heat resistance, it is inexpensive on the market, and hence, it has the merit of being capable of the easy manufacture of a large area substrate.
The laser annealing is known as a crystallizing technique which can crystallize the amorphous semiconductor film by giving high energy on only this film without considerably raising the temperature of the glass substrate. In particular, an excimer laser which emits light of short wavelengths at a large power is considered most suited for this use. The laser annealing with the excimer laser is carried out in such a way that a laser beam is worked by an optical system so as to define a spot or a line on a surface to-be-irradiated, and that the surface to-be-irradiated is scanned by the worked laser beam (i. e., that the projected position of the laser beam is moved relatively to the surface to-be-irradiated). The excimer laser annealing with, for example, the rectilinear laser beam is also capable of laser-annealing the whole surface to-be-irradiated by the scanning in only a direction orthogonal to the longitudinal direction of the surface, and it is excellent in productivity. It is therefore becoming the mainstream as the manufacturing technology of a liquid crystal display device employing TFTs.
The laser annealing is applicable to the crystallization of various semiconductor materials. So far, however, a high field-effect mobility has been realized by employing a crystalline silicon film for the active layer of each TFT. The technology has incarnated a liquid crystal display device of monolithic type wherein pixel TFTs constituting pixel portions, and the TFTs of driver circuits to be disposed around the pixel portions are formed on a single glass substrate.
However, the crystalline silicon film prepared by the laser annealing has been formed in the shape of the aggregate of a plurality of crystal grains, and the locations and sizes of the crystal grains have been random. It has accordingly been impossible to form the crystalline silicon film with the locations and sizes of the crystal grains designated. The interfaces of the crystal grains (grain boundaries) have involved causes for degrading the current transport characteristics of carriers under the influences of recombination centers and trapping centers ascribable to amorphous structures, crystal defects etc., and potential levels at the grain boundaries. Nevertheless, it has been next to impossible that a channel forming region, in which the property of a crystal affects the characteristics of the TFT seriously, is formed of a single crystal grain with the influences of the grain boundaries excluded. Until today, therefore, the TFT which employs the crystalline silicon film as its active layer has not attained characteristics comparable to those of a MOS transistor which is fabricated on a single-crystal silicon substrate.
As a method for solving such a problem, it is considered an effective expedient to enlarge the crystal grains and to control the locations of the large crystal grains, thereby to eliminate the crystal grain boundaries from the channel forming region. By way of example, xe2x80x9cLocation Control of Large Grain Following Excimer-Laser Melting of Si Thin-Filmsxe2x80x9d, R. Ishihara and A. Burtsev, Japanese Journal of Applied Physics, vol. 37, No. 3B, pp. 1071-1075, 1988, discloses a method which realizes the location control of crystals and the enlargement of grains by controlling the temperature distribution of a silicon film in three dimensions. According to the method, a film of high-fusing metal is formed on a glass substrate, the metal film is overlaid with a silicon oxide film which partially differs in thickness, and an amorphous silicon film is formed on the surface of the silicon oxide film. It is reported that crystal grain diameters can be enlarged to several xcexcm by irradiating both the surfaces of the resulting substrate with excimer laser beams.
The Ishihara et al. method features that the thermal characteristics of the base material of the amorphous silicon film are locally changed to control a heat flow to the substrate and to afford a temperature gradient. To that end, however, the three-layer structure of the high-fusing metal layer/silicon oxide layer/semiconductor film is formed on the glass substrate. It is structurally possible to fabricate a TFT of top gate type by employing the semiconductor film as an active layer. Since, however, a parasitic capacitance is incurred by the silicon oxide film interposed between the semiconductor film and the high-fusing metal layer, the power dissipation of the TFT increases, and the high-speed operation thereof is difficult of attainment.
On the other hand, the three-layer structure is considered to be effectively applicable to a TFT of bottom gate type or inverse stagger type by employing the high-fusing metal layer as a gate electrode. In the three-layer structure, however, even when the thickness of the semiconductor film is excluded, the total thickness of the high-fusing metal layer and the silicon oxide layer is problematic. More specifically, since a thickness suitable for the crystallizing process does not always agree with a thickness suitable for the characteristics of the TFT element, both the optimum design of the structure for the crystallizing process and the optimum design thereof for the element characteristics cannot be satisfied simultaneously.
Besides, when the high-fusing metal layer having no light transmissivity is formed on the whole surface of the glass substrate, a liquid crystal display device of transmission type cannot be fabricated. The high-fusing metal layer is useful in the point of a high thermal conductivity. Since, however, a chromium (Cr) film or titanium (Ti) film used as the high-fusing metal material exhibits a high internal stress, a problem will occur as to the close adhesion of the metal film with the glass substrate at a high possibility. Further, the internal stress may possibly exert influence even on the semiconductor film overlying the metal film and act as a force distorting the crystalline semiconductor film formed.
Meanwhile, for the purpose of controlling into a predetermined range a threshold voltage (hereinbelow, denoted as xe2x80x9cVthxe2x80x9d) which is an important characteristic parameter for the TFT, it has been required besides the control of the valence electrons of the channel forming region, to lower the charged defect densities of the base film and a gate insulating film which are respectively formed of insulating films in close touch with the active layer, and to consider the balance between the internal stresses of both the films. A material containing silicon as a constituent element, such as the silicon oxide film or a silicon oxynitride film, has been suitable for such requirements. Accordingly, the formation of the high-fusing metal layer for affording the temperature gradient is apprehended to collapse the aforementioned balance.
The present invention consists in techniques for solving the problems as stated above, and it has for its object to prepare a crystalline semiconductor film in which the location and size of a crystal grain are controlled, and to realize a TFT capable of high-speed operation by employing the crystalline semiconductor film as the channel forming region of the TFT. A further object of the present invention is to provide techniques by which such TFTs can be applied to various semiconductor devices of transmission type including a liquid crystal display device, an image sensor, etc.
Expedients for solving the problems will be described in conjunction with FIGS. 1A and 1B. Referring to FIG. 1A, a substrate 901 is overlaid with base films 902a, 902b, a semiconductor layer 903, a first protective insulating layer 904 and a thermal conduction layer 905. An insulating film, such as silicon oxide film or silicon oxynitride film, is well suited to each of the base films 902a, 902b. A light-transmissive material is employed for the thermal conduction layer 905. An amorphous semiconductor film, or a crystalline semiconductor film having a crystal structure is applicable to the semiconductor layer 903. Herein, the processing step of crystallization should most preferably be carried out by laser annealing. Especially, when an excimer laser emitting a laser beam at wavelengths of 400 nm or less is used as a light source, the semiconductor film can be preferentially heated, and hence, the use of the excimer laser is appropriate. A pulsed lasing type or a continuous emission type can be employed for the excimer laser. The light beam to irradiate the semiconductor layer 903 can be defined by an optical system into a rectilinear beam, a spot-like beam, a planar beam, etc., and the shape thereof is not restricted. Concrete conditions for the laser annealing shall be properly determined by a person who controls the processing step. It is to be understood that, at the crystallizing step in the present invention, the crystallization is effected by a meltingxe2x80x94solid phase reaction as will be outlined below.
With the laser annealing, the semiconductor film is heated and molten by optimizing the conditions of the irradiating laser light (or laser beam), so as to control the creation density of crystal nuclei and crystal growth from the crystal nuclei. In FIG. 1A, the semiconductor layer 903 is considered to be divided into parts 903a and 903b as indicated by broken lines. A region A in which the semiconductor layer 903a exists is a region in which the thermal conduction layer 905 is disposed, while a region B which corresponds to the semiconductor layer 903b is the remaining region which surrounds the region A. With the laser annealing in the present invention, both the region A and the region B are simultaneously irradiated with the laser beam. The pulse width of the excimer laser is from several nsec to several tens nsec, for example, 30 nsec. Therefore, when the frequency of pulsed lasing is set at 30 Hz for the irradiation, the semiconductor layers of the irradiated regions are heated by the pulsed laser beam for an instant, and they are cooled for a time which is much longer than the heating time. The semiconductor layers are brought into molten states by the irradiation with the laser beam. Immediately after the irradiation with the laser beam has ended, heat begins to diffuse toward both the side of the substrate 901 and the side of the first protective insulating layer 904, and the semiconductor layers are cooled to solidify gradually. The diffusion coefficient of heat differs depending upon substances. Whereas the thermal diffusion coefficient of a silicon oxide film is 0.04 cm2/sec, that of aluminum nitride employed for the thermal conduction layer 905 by way of example is 0.134 cm2/sec. In addition, the thermal diffusion coefficient of the air is 0.001 cm2/sec. Accordingly, the region A formed with the thermal conduction layer 905 is cooled quickly relatively to the region B.
The crystal nuclei are considered to be created and formed in the cooling process subsequent to the molten state. The creation density of the nuclei correlates with the temperature and cooling rate of the molten state. In this regard, it has been obtained as empirical knowledge that, when the high temperature is abruptly lowered, the nucleus creation density tends to heighten. Besides, in the process in which the molten state changes into a solid phase state, crystals grow from the crystal nuclei. In the case where the nucleus creation density is high, the crystal growth takes place from the individual crystal nuclei, and it stops at positions at which the grown ends of the crystals lie one upon another, whereby crystal grains and grain boundaries are formed. At the grain boundaries, an atomic arrangement is not kept orderly, and a large number of defect levels are formed. In such crystal growth, in the case of the high nucleus creation density, the crystals affect one another, and hence, only small crystal grains are formed. It is accordingly understood that the nucleus creation density needs to be lowered for forming large crystal grains.
In FIG. 1A, the crystal nuclei are created in the region A cooled from the molten state more abruptly, earlier than in the region B. Herein, the number of crystal nuclei to be created can be controlled to one by optimizing the area of the region A. Besides, the crystal growth from the crystal nucleus existing in the region A is preferentially done, whereby lateral crystal growth toward the region B proceeds in the subsequent cooling process. It is consequently permitted to enlarge one crystal grain around the region A.
The step of crystallization is not restricted to the laser annealing only, but thermal annealing and the laser annealing may well be combined therefor. By way of example, a crystalline semiconductor film can also be formed by first crystallizing an amorphous semiconductor film with the thermal annealing and thereafter irradiating the crystallized semiconductor film with a laser beam. A crystallizing method which employs a catalytic element may well be applied to the thermal annealing.
Defect levels at a density of 1016xcx9c1018/cm3 remain in the semiconductor film prepared in this way. It is therefore recommended to perform the processing step of hydrogenation by heat-treating the semiconductor film at a temperature of 300xcx9c450xc2x0 C. in a hydrogen atmosphere, a nitrogen atmosphere containing 1xcx9c3% of hydrogen, or an atmosphere containing hydrogen produced by forming a plasma. Owing to the hydrogenating step, the semiconductor film is doped with hydrogen on the order of 0.01xcx9c0.1 atomic %, and the defect level density can be lowered.
FIG. 1B shows a structure in which a substrate 901 is overlaid with a second protective insulating layer 906, in addition to base films 902a, 902b, a semiconductor layer 903, a first protective insulating layer 904 and a thermal conduction layer 905. The mechanism of crystal growth is similar to that explained with reference to FIG. 1A. The second protective insulating layer 906 is provided in order to control a cooling rate in a region B and to endow the region B with a thermal capacity.
At the crystallizing step as explained above, the thermal conductivities and thicknesses of materials to be used for the thermal conduction layer 905, the base films 902a, 902b, the first protective insulating layer 904 and the second protective insulating layer 906 are choices important for the purpose of controlling transient phenomena in the cooling of the semiconductor layer 903. The thermal conduction layer 905 needs to be made of a material whose thermal conductivity at the normal temperature is 10 Wmxe2x88x921Kxe2x88x921 or more. As such a material, it is possible to apply a material which contains one or more members selected from the group consisting of aluminum oxide, aluminum nitride, oxidized aluminum nitride, silicon nitride and boron nitride. Alternatively, it is allowed to apply a compound which contains Si, N, O and M (where letter M denotes at least one member selected from the group consisting of A1 and a rare-earth element).
On the other hand, the base films 902a, 902b, the first protective insulating layer 904 and the second protective insulating layer 906 are made of materials whose thermal conductivities at the normal temperature are lower than 10 Wmxe2x88x921Kxe2x88x921. An silicon oxynitride film should desirably be employed because of a material which has such a thermal conductivity, and which is suitable as the base layer of a TFT to be formed on a glass substrate. Of course, it is also possible to employ another material such as a silicon nitride film or a silicon oxide film. As the most preferable material, however, the first insulating film 902a or the second insulating film 902b may be formed of an silicon oxynitride film which is prepared from SiH4 and N2O by plasma CVD, and whose composition is set at an oxygen concentration from 55 to 70 atomic % and a nitrogen concentration from 1 to 20 atomic %.
The thermal conduction layer 905 is formed in an insular shape in conformity with the arrangement of the active layer of the TFT (the semiconductor film in which a channel forming region, a source region, a drain region and an LDD region are formed). Although the size of the thermal conduction layer 905 is set in conformity with, for example, the size of the TFT, it may be set at 0.1xcx9c10 xcexcm2 or so. The thermal conduction layer 905 is formed considering, at least, the position and size of the channel forming region of the TFT, whereby the channel forming region can be formed of the single crystal grain of the crystalline semiconductor film. That is, a semiconductor device according to the present invention can be endowed with a structure comparable to one in which the channel forming region is substantially made of a single-crystal film. Besides, when the thermal conduction layer 905 is removed after the crystallization by the laser annealing, the TFT or the like can be completed without any influence thenceforth.